Polymerization shrinkage of curable material is referred to the dimensional contraction during polymerization prior to the cured objective is developed. The covalent bond formation during polymerization bring monomer molecules closer than what they were in the normal van der Walls distance. This is the origin of polymerization shrinkage and it is also the origin of polymerization stress. Of course, the stress accumulation depends on how the materials are cured, that is, the polymerization kinetics.
The chemical structure of a curable resin determines almost every property aspects for any cured objectives in certain extend. Then it comes with the process or technology through which the curing proceeds. Formulation is a process primarily regarding as a balance between individual ingredient and acceptable property by adjusting the composition. A process that integrates all components together should be included in the formulation stage as well. Other emerging parameters involved during the polymerization process such as curing light intensity and curing time and curing mode, definitely would affect any property associated the polymerization like shrinkage, stress and mechanical property. In this invention, only composition formulation part is covered. More particularly it regards new resin development and composite formulation thereafter.
It is well known that with increasing molecular weight, the mobility of polymeric chain would be limited, the diffusion is becoming the rate control factor. In addition, such a limited mobility in a cross-linking system appear to come earlier in comparison with linear system, which means extra reaction would lead to an increasing polymerization stress. There are different ways to control the stress generation and development:
1. Limit polymerization rate;                Introducing a special rate controller like stable radicals;        Creating different polymerization zones from which the stress developed in a polymerized zone could be transferred to its adjacent unpolymerized zone and got relief like segmental polymerization technique;        Employing different polymerization groups;        Using macromonomer to limit its reactivity at the early stage;        
2. Limit polymerization conversion;
3. Limit cross-link density;
To reduce polymerization shrinkage and stress in the dental restorative composite, all of above approaches are taking into account as regards of chemistry approach. Besides, there is significant advance in the aspects of filler since it is composed of 60-90% in the entire composite. Increasing filler loading would lead to increasing in mechanical strength and reduction in polymerization shrinkage. Furthermore, the nature of filler, such as chemical composition, particle size and size distribution, surface character, silanization degree et al, have also demonstrated a tremendous impact on the balance between mechanical strength and shrinkage.
There is increasing demand for low shrinkage dental composite, since it was suggested that the lower polymerization shrinkage, the lower curing stress, then the higher clinically success in tooth restoration. However, such a correlation is not always true, this recommendation should be cautions. It is known that such recommendations for dental materials and clinical application techniques are frequently based on laboratory tests. However, if the lab test were based different methods, the recommendation would not make any sense. More specifically at the time being there is no standard method to evaluate the shrinkage and stress for dental materials, it should not be surprised to question any recommendation for particular dental material or product. Low shrinkage does not necessary grantee you low stress and less failure if the clinical operation is not proper, that it still quite technique sensitive procedure, not every clinician do it right. Just as an example, a new low shrinkage resin builds the foundation to a low shrinkage composite, but that does not assure that a low shrinkage product because the formulation and other associated technology can make it happen. Otherwise, the low shrinkage resin only means a good paper or paten, That is all. Same logical could be applied to tooth restoration with low shrinkage or even zero shrinkage composite, which is the base for a successful restoration but does not guaranteed it because it need highly trained clinician make it happen.
Polymerization shrinkage measurement is critical during low shrink material development, because it is important for establishing a reliable correlation between shrinkage and stress. It also helps for a fair judgement on low shrinkage composite to either clinician as dental material researchers. Unfortunately, there is no standard method by which polymerization shrinkage for resin or composite can be examined. Mercury dilatometer and gas pycnometer is employed in this laboratory to evaluate the polymerization shrinkage of resin and composite.
There are two different approaches to limit polymerization shrinkage and stress: chemical approach and technology approach. For light curable dental composite for instance, the chemical approach include new curing groups, new structural frames, new photoinitiator, new reaction kinetics, new coupling agent for new interface interaction between resin and fillers, and new filler et al; and technology approach includes: new curing light source, new curing energy, new curing mode, new technique to create a cavity, new technique to fill the cavity et al. All of these processes determine the shrinkage and stress and their development, which are believed to be associated directly to a failure restoration.
This invention involves a chemical approach to limit polymerization shrinkage and stress. More particularly it regards a new resin and its composition development. In this invention, therefore, a general method is presented to make a polymerizable single net, such as a polymerizable macrocyclic oligomer, from which a 3D network would be developed via less direct polymerization of (meth)acrylate. Now the whole picture is clear: to pre-build a polymerizable macrocyclic as single net outside the tooth cavity first, then assembly it into a network inside the filled cavity with limited reaction. As a result for this new approach, the total shrinkage would be reduced due to the limited reaction group. However, the necessary mechanical property would not be significantly impaired because the cyclic nature can make easy in cross-link density development. In addition, a new mono(meth)acrylate with bulky side group was combined with the macrocyclic resin to generate a resin system that afford better balance regarding mechanical strength, polymerization shrinkage ands contraction stress. Finally a proper glass filler composition is also presented which determine the mechanical strength and handling property as well.
Cyclic and Macrocyclic Oligomers vs. Polymerizable Macrocyclic Oligomers
Various macrocyclic oligomers are well investigated since the researchers at GE developed a new approach to prepare cyclic carbonate oligomers. For example, in U.S. Pat. No. 4,644,053, it was disclosed a method to synthesize single macrocyclic compounds. Then various macrocyclics oligomers, including carbonates, esters, amides, ethers, imides, sulfides, et al, have been prepared. However, high temperature ring-opening reaction has to be involved to convert these macrocyclics into high molecular weight polymers. None of them could be further polymerizable without ring-opening.
Many photopolymerizable resins have been developed from mono-, di- or multiple functional resins to dendrimer, but no macrocyclic oligomer with multipolymerizable groups has been reported: U.S. Pat. No. 5,047,261, disclosed a composition containing a five-member carbonate cyclic group for fast copolymerization with mathacrylate.
U.S. Pat. No. 5,962,703, disclosed functionalized bicyclic methacrylate with norboneyl or norbonadienl group. U.S. Pat. No. 5,792,821, disclosed polymerizable cyclidextrin (CD) derivatives, in which various methacrylate was attached on CD. More recently, U.S. Pat. No. 6,043,361, disclosed polymerizable cyclic allylic sulfides is used for low shrinkage materials. All of these cyclic-related new resins are limited to small cyclic sizes that are exclude in the scope of this invention.
The occurrence of cyclization reaction is favorite at high dilution condition. However, its efficiency limits its possible application in commercial development. Fortunately a pseudo-high-dilution technology was developed to solve this problem. This technique was adopted here to prepare a polymerizable macrocyclic oligomers. More specifically, a free-radically polymerizable macrocyclic oligomers are prepared under pseudo-high-dilution condition via a condensation reaction between a reactive and free radical polymerizable precursor and various coupling agents. With such a method, various macrocyclics could be formed via any linkage to afford carbonate, ester, siloxane, phosphonate, and et al derivatives. On the other hand, the condensation groups usually have to be activated to assure a mild reaction for cyclization with the coupling monomers in order to avoid any premature polymerization of the pre-attached methacrylate groups. Typical reaction scheme is illustrated as following:
                A: any aromatic or aliphatic or the combination moiety;        B: any linkage such as ether, thioether, ester, amide, carbonate, urethane, and urane, et al;        X: any reactive group such as hydroxyl, carboxyl, et al        Z: polymerizable groups like (meth)acrylate, vinyl, vinyl ether, and epoxy, et al        R: any aromatic or aliphatic or the combination;        Y: any activated groups such as acylidied, acylamide, formated, carbonamade;        D: any of aromatic or aliphatic or their combination moiety;        
The reactive monomer can be synthesized or commercially-available; It may not contain the primary polymerizable groups but the coupling agent must have at least one such a polymerizable group to ensure the formation of resulting ,macrocyclic oligomer to be further free-radical polymerizable.
                Y: Ar, cyclohexyl,        X: O, COO,        

BisGMA is one of widely used dental resin and it contains two free radical polymerizable group, methacrylate and two hydroxyl groups. This turns BisGMA an ideal candidate for polymerizable macrocyclic oligomer, although the presence of BisGMA isomer would make more complicated to this approach. As shown in Scheme II, carbonyldiimidazol (CDI, 1), was used to selectively reacted with the secondary alcohol in BisGMA (2) to give an activated BisGMA, DIZ-BisGMA (3). It was isolated and the chemical structure of DIZ-BisGMA was fully characterized with FITR and NMR. According to the recent report by Davis et al, CDI and its intermediates could exhibit surprisingly specificity towards primary, secondary, tertiary functional groups, of the same type, during the controlled formation of various well-defined molecular sequences [1-5]. In this invention, our idea is to adopt same chemistry of CDI and to selectively activate the two secondary hydroxyl groups in a free-radically polymerizable diol, BisGMA. Furthermore, the resulting precursor, DIZ-BisGMA, was made to react with various primary diols under a pseudo high-dilution condition, as shown in Scheme III, to generate macrocyclic carbonate oligomer bearing multiple polymerizable methacrylate groups. The two reactants were charged into the system in a high-dilution condition via two liquid pumps with slowly, precisely controlled addition in order to ensure a favorable formation of cyclic product. Actually cyclic product is accumulated within the reaction system and the final concentration can reach 0.02M, which is much higher than the classical high dilution condition (0.001M). However, the key to this procedure is to maintain a low initial concentration of reactants by controlled feeding. Therefore, it is referred as pseudo-high-dilution (PHD) method. The following examples will present the detailed procedure of the preparation of various precursors, macrocyclic oligomers, new cyclic diluent and composites thereafter.